Given data:
Suppose two sides of the right angle triangle are a=3 meter and b=4 meter and in order to calculate the longest side, the expression for the Pythagoras theorem is,

Substitute 3 meter for a, and 4 meter for b in the above expression.

Here, consider only positive value of c because length can't be negative.
Thus, the Pythagoras theorem is used to find the missing side of the right angle triangle when any two sides are given.